The development of photolithography in 1959 ushered in a new age for the electronics industry. Computers began to shrink from auditorium-sized behemoths to desktop-sized personal computers with accompanying increases in actual speed and computing power. The crown jewel of this revolutionary technological feat was, and still is, the microchip. Fabricated out of silicon and etched and coated using optical masks, the microchip changed the way engineers and scientists viewed the need for physical-device size in relation to actual task requirements. For example, adding machines and calculators were no longer viewed as having to have large keyboards and complicated mechanical apparatus, as well as accompanying hefty physical mass. The ubiquitous hand-held calculator is one result of this change in viewpoint. Occurring concurrently with this revolution in electronics technology, and mostly because of it, miniaturization of many other technologies started taking place, and currently, is revolutionizing these technologies as well. One of these technologies involves sensors; i.e., devices which measure either qualitatively or quantitatively some attribute of their environment. Examples of such devices are temperature sensors, motion sensors, magnetic-field detectors, radiation detectors, pressure sensors, and chemical sensors, to name a few. Miniaturizing their functions has made them not only smaller, so that they can be used in situations previously not thought possible, but has in many cases made them better able to perform their intended tasks as well.
As miniaturization of mechanical and electrical systems occurs, the role of physical and chemical effects and parameters have to be reappraised. Some effects, such as those due to gravity or ambient atmospheric pressure, are relegated to minor roles, or can even be disregarded entirely, while other effects become elevated in importance or, in some cases, actually become the dominating variables. This "downsizing reappraisal" is vital to successful miniaturization. In a very real manner of speaking, new worlds are entered into, in which design considerations and forces that are normally negligible in real-world applications become essential to successful utilization and application of the miniaturized technology.
Surface tension and the closely-related phenomena, wettability, are usually not comparable in effect to normal physical forces at macroscopic levels. For example, surface tension is usually ignored when determining fluid flow through a pump or tube, its effect is many orders of magnitude smaller than pressure drop caused by viscosity. That is because difference in pressure, .DELTA.P, existing between the inside of a droplet and the outside is given by the relationship EQU .DELTA.P=2.gamma./r
where .gamma. is surface tension and r is droplet radius. Normally, in most macroscopic applications, droplet dimensions are measured in thousands of microns. Pressure differences due to surface-tension effects are therefore inconsequential, typically measuring far less than atmospheric pressure. For comparison, pressure drops resulting from viscous flow are typically on the order-of-magnitude of tens of atmospheres. When r is on the order of microns, however, pressure differences becomes enormous, frequently surpassing hundreds of atmospheres.